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Date: Sat, 17 Dec 2011 22:30:33 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>, Andrew Morton <akpm@linux-foundation.org>,
        Kay Sievers <kay.sievers@vrfy.org>, Namhyung Kim <namhyung@gmail.com>,
        Lukas Czerner <lczerner@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutex
 recursive locking
Message-ID: <20111217223033.GB2203@ZenIV.linux.org.uk>
References: <20111217215333.GA3313@swordfish>
 <20111217221232.GA2203@ZenIV.linux.org.uk>
 <20111217221928.GB3313@swordfish>
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On Sun, Dec 18, 2011 at 01:19:28AM +0300, Sergey Senozhatsky wrote:

> Sorry, why is that a false positive?
> 
> blkdev_put() calls lo_release() while holding bd_mutex,
> lo_release() calls loop_clr_fd() -> fput(). fput() once again
> attempts to grub already held bd_mutex calling blkdev_put().
> Looks like a recursion to me.

Because of this:
        /* Avoid recursion */
        f = file;
        while (is_loop_device(f)) {
                struct loop_device *l;

                if (f->f_mapping->host->i_bdev == bdev)
                        goto out_putf;

                l = f->f_mapping->host->i_bdev->bd_disk->private_data;
                if (l->lo_state == Lo_unbound) {
                        error = -EINVAL;
                        goto out_putf;
                }
                f = l->lo_backing_file;
        }
in loop_set_fd().  Think of it for a minute - if we could run into the
same bdev in that recursion, what would have happened on read() from
that sucker?  So yes, it is a false positive.  And your patch would
simply leave the underlying device opened, with all the consequences...